1. Field of the Invention
The present invention relates generally to an electronically controlled automatic transmission and, more particularly, to an improved automatic transmission coastdown and coastdown/tip-in control sequence method.
2. Discussion
Automotive vehicles generally incorporate a motive force system having three basic components: an engine, a power train and wheels. The engine produces force by converting chemical energy from a liquid fuel into the mechanical energy of motion. The power train transmits the resultant force of this kinetic energy to the wheels which frictionally contact a surface for moving the vehicle. The main component of the power train is the transmission, which transmits engine torque over a relatively limited angular speed range to the wheels over a broader speed range, in accordance with the tractive-power demand of the vehicle. The transmission also controls the direction of rotation applied to the wheels so that the vehicle may be driven both forward and backward.
One advanced type of transmission is a four speed electronically controlled automatic transmission with overdrive. Examples of this type of electronically controlled automatic transmission are described in U.S. Pat. No. 4,875,391, entitled "An Electronically-Controlled, Adaptive Automatic Transmission System", issued on Oct. 24, 1989 to Leising et al.; U.S. Pat. No. 4,905,545, entitled "Method Of Controlling The Speed Change Of A Kickdown Shift For An Electronic Transmission System", issued on Mar. 6, 1990 to Leising, et al. and U.S. Pat. No. 4,951,200, entitled =37 Method Of Controlling The Apply Element During A Kickdown Shift For An Electronic Automatic Transmission System", issued on Aug. 21, 1990 to Leising, et al. These patents are owned by the Assignee of the present application and are incorporated herein by reference. However, it should be appreciated that the principles of the present invention are not limited to any particular electronically controlled automatic transmission, and that the present invention may be applied to a wide variety of other powertrain configurations.
The transmission in the patents incorporated herein by reference includes four friction elements or clutches which are applied or engaged in various combinations in relation to each of the vehicle's gears. Those elements in the present transmission include an underdrive clutch (applied in first, second and third gears), an overdrive clutch (applied in third and fourth gears), a two/four shift clutch (applied in second and fourth gears) and a low/reverse clutch (applied in first and reverse gears).
To apply each of these clutches, an electronically controlled hydraulic fluid actuating device such as a solenoid-actuated valve is used. There is typically one valve for each clutch: an underdrive clutch solenoid-actuated valve, an overdrive clutch solenoid-actuated valve, a two/four shift solenoid-actuated valve and a low/reverse solenoid-actuated valve. Each valve controls fluid flow to a respective clutch apply cavity. The flow of fluid into a clutch apply cavity causes a piston to move axially, which results in the application or engagement of that clutch. Fluid flow is enabled by the opening of the solenoid-actuated valve in response to command or control signals received by the solenoid from an electronic control system.
The electronic control system includes a microcomputer-based transmission control module capable of receiving and monitoring input signals indicative of various vehicle operating conditions such as engine speed, torque converter turbine speed, output speed (vehicle speed), throttle angle position, brake application, predetermined hydraulic pressures, a driver selected gear or operating condition (PRNODDL), engine coolant temperature and/or the ambient air temperature. Based on the information contained in these signals, the controller generates command or control signals for causing the actuation of each of the solenoid-actuated valves which regulate the application and release of fluid pressure to and from the apply cavities of the clutches or frictional elements of the transmission. Accordingly, the controller is programmed to execute predetermined shift schedules stored in a memory of the controller through appropriate command signals to the solenoid-actuated valves.
Of these shift schedules, transmission downshifts (from a higher gear into a lower gear) in this type of system, other than driver-actuated manual downshifts in which the driver physically changes the position of the vehicle shift lever, can be generally classified into two basic types: a power-on kickdown shift, made in response to a sharp increase in throttle to add torque, and a coastdown shift, which occurs as the vehicle is being slowed to a stop. Conventional coastdown shift control in these transmissions has in the past been essentially kickdown control that was adjusted where possible for the low speed and torque conditions of coastdown shifts. The control strategy was primarily based on control of the release element, (i.e., the clutch to be vented or released in performing the gear change). In this type of control, the release element remained engaged throughout the downshift. The turbine speed was allowed to rise to slightly above a target gear turbine speed and was controlled at that level until the apply element engaged. Only after the apply element was engaged would the release element be completely vented or "dumped".
This type of control strategy protects the transmission against the throttle re-opening at any time, since the release element remains engaged until the apply element has been engaged. However, this strategy can result in enough simultaneous over-capacity of the applying and releasing elements to cause a fight that results in a condition sometimes referred to as "coastdown bump". This has been up to now almost unavoidable because the exact timing of the oncoming element's application was unpredictable since this timing may vary with transmission oil temperature, pump and valve clearances, engine idle speed and other such variables.